Flexible polyurethane foam-made seat pads used for seats mounted on vehicles and conveyances, such as boats, ships, and aircraft, furniture chairs, and the like may give users a sense of lateral wobble. For example, a seat pad may be deformed by vibrations in a low-frequency band (for example, about 1 Hz) input when a vehicle goes around a mild curve or makes a lane change, resulting in production of a sense of wobble, such as sideslip or lateral rocking about a roll axis. Such a sense of wobble is a factor affecting the steering stability. There is a technique for setting the hardness of a seat pad in order to reduce such a sense of wobble (Patent Literature 1).